Re: Hilbert System Proof. ( p /\ q ) -> (q /\ p )

Plato is right: you need to say what Theorems 3.23 and 3,25 are and what is the definition of /\. I'll venture a guess that p /\ q is ~(p -> ~q), Theorem 3.23 is q -> ~~q and Theorem 3.25 is (p -> q) -> (~q -> ~p). Then the derivation would be as follows.